1. Field of the Invention
The present invention relates to oilfield tubular threaded connections and, more particularly, to a highly reliable connection with tapered threads and a torque shoulder. A threaded oilfield tubular connection is provided with features that result in increased compression and torsional capacity, and more particularly a tubular connection which limits the loss of preload.
2. Description of the Related Art
A common problem associated with oilfield tubular connections is the effect of compression on the connection and the resulting loss of preload. While many connections are able to withstand a high compression loading, a good portion of preload may be lost when the compressive force is lowered, resulting in leakage of the connection. A common oilfield connection includes a two-step axially spaced threads on different diameters, and an intermediate torque shoulder between the step threads.
Non-upset, integral connections, such as flush and semi-flush outer diameter connections, have the inherent limitation of the wall thickness of the pipe upon which they are machined. Flush connectors have the same outer diameter as the pipe. Semi-flush connectors are also referred to as expanded box connectors, since the pipe is expanded on the end upon which the female (box) connector is machined. Swaging and cold forming, unlike upsetting, may move the wall to a different position, but do not significantly affect the thickness. The connection designer is frequently faced with the need for balancing many features. This includes a seal or seals, robust thread form, and a place to store preload torque, most often in the form of a torque shoulder. Accommodating these features in a limited space while maintaining connection capacity is a challenge.
Several integral, two-step, negative load flank, tapered, dual seal connectors exist in the market. The GP ANJO includes a negative load flank thread form, two step, tapered thread pitch, 15 degree torque shoulder angle, internal seal, and an external seal at box face. The Hydril SLX has a negative load flank thread form, two step, tapered thread pitch, 15 degree torque shoulder angle, internal seal, and external seal mid-connector on a small step. The VAM SLIJII connection has a negative load flank thread form, two step, tapered thread pitch, vertical torque shoulder, internal seal, and external seal at box face. An integral, single-step, negative load flank, tapered, single seal connector is also marketed. The Hydril 523 connection has negative load flank dovetail thread form, negative stab flank dovetail thread form, single step, tapered thread pitch, torque shoulder achieved by simultaneous engagement of the load and stab flank, internal seal, and an external seal provided by thread compound and thread profile. These connectors are known as wedge threads due to their use of the stab flank as a stop shoulder.
Threads which have load and stab flank engagement in the final made up position already exist. These include API connectors including LTC and STC commonly referred to as eight-round. These are known as flank to flank threads. Grant Prideco's TC-II connector product line has load and stab flank engagement. These threads have largely been abandoned for use in integral connectors due to jump out. Jump out is the phenomena particular to more flexible connectors, such as the integral joints. When these connectors experience high tension loads, the threads disengage due to the radial reaction force on the load flank and come apart. Negative load flank forms resolve this problem.
A common thread form involves root to crest contact. This is common to most negative load flank threads as well as some API connectors including the buttress thread form. The absence of a negative load flank makes this form less desirous for integral joints. The negative load flank and the root to crest thread form, in absence of a wedge, is the most common thread form for high performance integral joints. The weakness of this thread form is in the stab flank clearance. This is necessary to prevent galling, but limits its capacity in compression, bending and torsion.
Finally, wedge threads use a different load and stab flank lead to enact both flank to flank and/or root to crest engagement during makeup while avoiding galling with either positive or negative flank angles. A weakness of this thread form is the lack of makeup repeatability as to its position and the effect on metal to metal seals. This may be compensated for with long, low taper seals that have lower variability in interference with higher variability in axial position. High compression, bending and torsional strength are attained through the helical compression flank engaged simultaneously with the load flank, along with the negative load flank angle to avoid jump out and the option for the negative stab flank for additional radial support.
The disadvantages of the prior art are overcome by the present invention, and an improved oilfield tubular connection which utilizes an improved torque shoulder to limit the effective compression on loss of preload is hereinafter disclosed.